This invention generally pertains to apparatus for generating an impulse mechanical force as between two members using electrical input energy. More particularly, the invention provides a configuration for an elastomeric composite boot comprised of two interconnected juxtaposed elastomeric members, each member including a plurality of serially-connected electrical conductors embedded within the composite forming the boot members. The electrical conductors are of such configuration and orientation within the boot as to effectively generate a very high proximal mechanical force as between the two juxtaposed members. The force effects an impulse separation of one member from the other when an electrical impulse current is applied to the conductors. The generated electro-expulsive force is useful in many and various applications where an impulse mechanical motion is desired.
A particular application which benefits from an impulse mechanical force pertains to aircraft deicing of critical surface areas. Aircraft icing problems are generally associated with airfoil, rotor blade, and engine inlet leading edge surfaces. U.S. Pat. No. 4,690,353 to Leonard A. Haslim and Robert D. Lee and assigned to the United States of America as represented by the Administrator of the National Aeronautics and Space Administration traces the history of attempts to solve the aircraft icing problem. This patent describes an electro-expulsive system which also attempts to address the aircraft icing problem. According to this patent, two flexible ribbon-type electrical conductors are mounted in very close proximity to each other within respective upper and lower elastomeric members and an electrical impulse current applied to the conductors results in opposing fields which interact in such a manner as to force a separation of the two members. The generated force F(1) for a single conductor configuration is given in the patent as: EQU F=4II'a.sup.-1 [tan.sup.-1 ab.sup.-1 -1/2ba.sup.-1 ln (1+a.sup.2 b.sup.-2).times.10.sup.-7 N/m
where
F is the force per unit length, PA0 I is the current in one conductor in a first direction, PA0 I' is the current in the other conductor in an opposite direction PA0 "a" is the width of the conductors, and PA0 "b" is the distance separating the parallel conductor segments. Obviously, the force "F" may be increased by increasing the current. PA0 an elastomeric ply which defines a first member; PA0 an elastomeric ply which defines a second member in juxtaposition with respect to the first member and both members have interconnected peripheral edges to enclose a space separating the two members; PA0 a release membrane within the space between the two members to maintain the separate integrity of each of the members; and PA0 an electrically-conductive element embedded within the elastomeric material comprising each of the members, the element oriented such as to pass from the first member, through the peripheral edge between the members, and into the second member in a substantially continuous manner between the members such that a plurality of contiguous segments of the electrically-conductive element are evident within each of the members and the application of a particular magnitude electrical current pulse generates an electromagnetic force in the first member which opposes an electromagnetic force in the second member and the two members are impulsively separated by the opposing forces.
The above-referenced patent cites an example wherein a force F=2430lbs/ft (35.5.times.10.sup.3 N/m) may be obtained when the two electrical ribbon conductors each have a width of 0.300 inches (7.62 mm), are separated by a gap of 0.079 inches (2.0 mm), and an electrical current of 3000 amperes is applied.
The present applicant, in attempting to fabricate a deicing boot for a jet aircraft engine inlet, could not obtain the results cited in the U.S. Pat. No. 4,690,353. Experiments conducted on a boot made in accordance with the teachings of the cited patent were marginal in generating enough force which would break up ice on the boot surfaces. It seemed apparent that very little of the available electrical input energy was being converted efficiently to mechanical energy sufficient to displace the members comprising the boot structure. Only substantial increases in the current would improve the performance.
In view of these failures, the teachings of the above-cited prior art patent were more closely scrutinized. Upon applying the specific data for the example of an electro-expulsive boot to the given force equation, it was found that only slightly over 450 N/m could be obtained from the calculations. This is in contrast to a stated force of 35.5.times.10.sup.3 N/m. This discrepancy resides in the fact that the radian term in the arc-tangent function was inappropriately evaluated in the patent calculations.
The serpentine conductor configuration taught in the above-cited patent was also more critically assessed and it was determined that it was questionable whether such configuration could produce sufficient energy conversion efficiency to generate the high mechanical force necessary in a deicing application. This assessment was taken from the fact that, while a pair of vertically positioned and separated ribbon conductors generated a repelling force between themselves when their respective currents are in opposite directions, there are other pairs of contiguous ribbon conductors in the immediate proximity to the first pair which have currents which generate attractive forces between opposing members of adjacent pairs. In this circumstance and before any useful electro-expulsive force is generated, the attractive forces which exist must be overcome. This is obviously an inefficient energy conversion configuration.
Upon re-thinking the physical phenomena as applied to electro-expulsive boots, the applicant determined that an improvement in mechanical output force may be possible by serially connecting multiple conductors so that the same available current flowed in the same direction through contiguously-positioned conductors. In this way the generated field which exists between any two conductors in the same boot member will be additive. When the upper and lower members of a boot structure each contain multiple conductors, the overall opposing fields which are generated in the boot members greatly increases the mechanical output force. Accordingly, the force equation for this configuration was determined to be N-squared times the force F(1) for a single turn configuration i.e., F(N)=N.sup.2 .times.F(1) where (N), N and (1) denote the number of turns. From this it becomes apparent that, for the same applied current, the force is also dependent upon the square of the number of conductors rather than just the available current as taught by the prior art.
In accordance with the above realization, an experimental boot was fabricated using multiple turns of #14 magnetic wire conductors. The conductors were laid up in a serial arrangement such that ten turns of wire were evident in each of the upper and lower boot members. Upon application of an electrical impulse current, sufficient mechanical output force was generated to propel a fourteen inch long 2.times.4 board approximately five feet in the vertical direction. This result was obviously a remarkable advancement over a state of the art boot which could only propel a few coins a couple of inches upwards!
It will be appreciated that, subsequent to prototype testing, analytical development of the dynamic equations of the above-described system revealed why implementing a multi-turn boot configuration has a beneficial effect. Not only are the forces on the conductive elements increased with an increase in the number of conductor turns, but, in addition, the electrical-mechanical conversion efficiency is enhanced. By application of a multi-turn conductor configuration, the variable (N) for the number of conductor turns can be used to optimize the conversion efficiency. Further, it can be shown that the separation distance between upper and lower boot members is not significant when the ratio of the boot area to the upper-lower member separation is reasonably large. In other words, the forces and pressures generated are independent of the separation dimension and these will not vary as the separation between the two conductors is varied. This is remarkable in view of the prior art which teaches otherwise. Furthermore, this is very significant in a multi-turn electro-expulsive boot in accordance with the present invention as it allows for vertical stacking of electrical conductors without losing output force due to any separation constraint imposed on the boot structure.